(1) Field of the Invention
The present invention relates to a compound helicopter with a pair of tail booms on the basis of a compound helicopter featuring vertical take-off and thrust capabilities and more specifically, the invention relates to a compound helicopter with a pair of tail booms on the basis of a compound helicopter with a main rotor providing lift during vertical and forward cruise flight operations, a pair of fixed main wings and a pair of additional propulsive devices for lift and thrust during forward cruise flight.
(2) Description of Related Art
The dominant helicopter configuration in the present time is based on Sikorsky's basic design with a main rotor and an auxiliary tail rotor to counter torque. Said conventional helicopters show excellent hover capabilities but suffer from important limitations in terms of horizontal flight speed. These limitations are associated to two aerodynamic phenomena at the main rotor: the retreating blade stall and the maximum blade tip velocity. In general terms, the lift and thrust force capabilities of a helicopter's main rotor decrease with increasing forward speed.
Regarding the increasing demand for aircrafts with vertical take-off capabilities and high-speed and/or high-range capabilities, numerous attempts have been done to combine the efficiency and performance of fixed-wing aircrafts in forward flight and the advantageous hover and vertical take-off capabilities of conventional helicopters. These attempts have been translated in a wide and diverse variety of aircraft configurations. Compound helicopters and convertiplanes are basically the most relevant concepts aiming to overcome the horizontal flight deficiencies of pure helicopters by introducing attributes of fixed-wing aircrafts to helicopters. However, said concepts represent a compromise between conventional helicopters and fixed-wing aircrafts. Said concepts have always to be conveniently adapted to the planned aircraft's mission profile.
Compound helicopters are characterized by a lift compounding, a thrust compounding or by a combination of both and basically aim to off-load the rotor from its simultaneous lifting and propulsive duties to allow for higher forward speeds.
A lift compounding entails adding wings to a pure helicopter hence enabling to increase the load factor of the helicopter and to reach a higher maneuverability. This improves the efficiency of the helicopter at moderately high speeds but at the expense of reduced efficiencies at lower forward speeds and in the hover.
A thrust compound implies the addition of auxiliary propulsion devices to helicopters. This has been typically accomplished either by means of a single or a pair of propellers being driven by driveshafts powered by the main turboshaft engines or by the use of additional engines.
A more extended configuration of a compound helicopter includes both the addition of wings and propulsion units. The lift during cruise is simultaneously provided by the main rotor—in powered condition or in autorotation modus—and wings. The compound helicopter hence overcomes the rotor lift limits by means of the wings and the rotor thrust limits by means of the propellers. As a result, the benefit of a higher load factor is obtained along with potential for higher speed. The use of a pair of thrust propellers enables for a simultaneous torque correction.
A wide variety of said compound architectures have been proposed and some of them developed to a more or less level of maturity but never reaching serial production. A typical configuration with two wing-mounted propellers is described in U.S. Pat. No. 3,105,659, US 2009/0321554 and U.S. Pat. No. 6,513,752. The propulsion device can be a single nose-mounted propeller as disclosed in US 2005/0151001 or a single rear-mounted propeller as disclosed in U.S. Pat. No. 3,241,791, CA 2316418. All said typical configurations feature a pair of main wings located below the main rotor.
A compound helicopter based on a tandem wing configuration, meaning the arrangement of a pair of wings on both of the helicopter's front end and aft end, has been proposed in US 2011/0114798, the propulsion devices being arranged on the front wings. This configuration is characterized by propulsion propellers operating in the turbulent downwash from the main rotor which results to a reduced efficiency and increase noise generation. The location of unducted wing-mounted propellers in the cabin area leads furthermore to serious penalties in terms of passenger safety. In case of a blade separation, the blade has to be retained by additional structural features to avoid a cabin penetration which additionally increase the structural weight. Open rotors being placed close to the doors represent further serious safety penalties during boarding and increase the noise exposure in the cabin.
Typically, the architecture of the aforementioned helicopters with a single main rotor features a main body comprising the cabin, the main rotor and the powerplant with engines, gear box and transmission being placed somewhere on top of the main body above the cabin. The center of gravity is located close to the rotor mast. Since the distribution of payload, passengers and fuel influence the mass distribution and the balance of the helicopter, special care has to be paid to maintain its location within specific limits.
The document U.S. Pat. No. 4,589,611 A discloses a high wing, twin jet gyrodyne having air jet reaction, contrarotating rotors powered with bypass air from twin turbofan engines. Tail pipes from the turbofan engines are disposed parallel to and at opposite sides of a longitudinal duct for discharging bypass air rearwardly. Control valve means can control supply of bypass air selectively to the hollow rotor mast for driving the rotors and to the longitudinal air discharge duct. The aft portions of the tail pipes and of the air discharge duct are connected by a horizontal stabilizer and rudders are located in the slipstream discharged from the tail pipes and the air discharge duct. The wings have drop tip sections providing flotation outriggers in their downwardly projecting positions.
The document U.S. Pat. No. 3,448,946 A discloses a compound helicopter with a pair of tail booms, said compound helicopter comprising a fuselage with a cabin inside, a rear end and a front end. A longitudinal roll axis extends through the rear end and the front end, said roll axis and a pitch axis transversal to said roll axis defining an essentially horizontal plane (X, Y). Said pair of tail booms are longitudinal structures arranged respectively symmetric to opposed sides of the fuselage in the essentially horizontal plane (X, Y) and said tail booms are interconnected with each other at their respective aft ends by means of a horizontal tail plane. Said compound helicopter of the state of the art further comprises a power plant and drive lines. A main rotor is mounted onto the fuselage and driven by said power plant via said drive lines. One pair of fixed main wings is attached to the opposed sides on top of the fuselage in the essentially horizontal plane (X, Y). Two propulsive devices provide thrust and anti-torque. The tail booms are attached to the wing structure and parallel to the longitudinal fuselage axis. Or the tail booms are attached to the fuselage body and sections of the axis of the tail booms are slightly slanted. The propulsion device of said compound architecture is attached to the aft end of the fuselage. Despite the advantage of having undisturbed wings, this propulsion device does not allow for anti-torque capabilities, hence still requiring an additional tail rotor since a configuration with a single fixed pusher propeller, mounted at the aft end of the fuselage is not capable to provide anti-torque.
The document U.S. Pat. No. 3,409,248 A discloses an aircraft with two rotors carrying a wing.
The document U.S. Pat. No. 5,873,545 A discloses a combined flying machine with a fuselage in the form of a central thick wing with a vertical open tunnel, in which there is mounted a lifting rotor. Said machine is also provided with outboard wings and a tail unit. The machine is equipped with a landing device on an air cushion that surrounds an outlet from the tunnel. A power plant of said flying machine comprises two engine modules, disposed from two sides of the tunnel and connected with the lifting rotor and the propulsion propellers. The area of the tunnel cross-section in the plane of the lifting rotor rotation amounts to 0.3 to 0.8 of the area of the landing device air cushion, which ensures a safe landing of the flying machine on an unprepared landing site even with a failure of one of the engine modules.